Ligand binding and conformational states of photoprotein obelin

Many proteins require a non-covalently bound ligand to be functional. How ligand binding affects protein conformation is often unknown. Here we address thermal unfolding of the free and ligand-bound forms of photoprotein obelin. Fluorescence and far-UV circular dichroism (CD) data show that the various ligand-dependent conformational states of obelin differ significantly in stability against thermal unfolding. Binding of coelenterazine and calcium considerably stabilizes obelin. In solution, all obelin structures are similar, except for apo-obelin without calcium. This latter protein is an ensemble of conformational states, the populations of which alter upon increasing temperature

Bioluminescent and spectroscopic properties of His-Trp-Tyr triad mutants of obelin and aequorin.

Ca2+-regulated photoproteins are responsible for the bioluminescence of a variety of marine organisms, mostly coelenterates. The photoproteins consist of a single polypeptide chain to which an imidazopyrazinone derivative (2-hydroperoxycoelenterazine) is tightly bound. According to photoprotein spatial structures the side chains of His175, Trp179, and Tyr190 in obelin and His169, Trp173, Tyr184 in aequorin are at distances that allow hydrogen bonding with the peroxide and carbonyl groups of the 2-hydroperoxycoelenterazine ligand. We replaced these amino acids in both photoproteins by residues with different hydrogen bond donor–acceptor capacity. All mutants exhibited luciferase-like bioluminescence activity, hardly present in the wild-type photoproteins, and showed low or no photoprotein activity, except for aeqH169Q (24% of wild-type activity), obeW179Y (23%), obeW179F (67%), obeY190F (14%), and aeqY184F (22%). The results clearly support the supposition made from photoprotein spatial structures that the hydrogen bond network formed by His–Trp–Tyr triad participates in stabilizing the 2-hydroperoxy adduct of coelenterazine. These residues are also essential for the positioning of the 2-hydroperoxycoelenterazine intermediate, light emitting reaction, and for the formation of active photoprotein. In addition, we demonstrate that although the positions of His–Trp–Tyr residues in aequorin and obelin spatial structures are almost identical the substitution effects might be noticeably different

Green-Fluorescent Protein from the Bioluminescent Jellyfish Clytia gregaria Is an Obligate Dimer and Does Not Form a Stable Complex with the Ca2+-Discharged Photoprotein Clytin.

Green-fluorescent protein (GFP) is the origin of the green bioluminescence color exhibited by several marine hydrozoans and anthozoans. The mechanism is believed to be Fo¨rster resonance energy transfer (FRET) within a luciferase-GFP or photoprotein-GFP complex. As the effect is found in vitro at micromolar concentrations, for FRET to occur this complex must have an affinity in the micromolar range. We present here a fluorescence dynamics investigation of the recombinant bioluminescence proteins from the jellyfish Clytia gregaria, the photoprotein clytin in its Ca2+-discharged form that is highly fluorescent (¿max = 506 nm) and its GFP (cgreGFP; ¿max = 500 nm). Ca2+-discharged clytin shows a predominant fluorescence lifetime of 5.7 ns, which is assigned to the final emitting state of the bioluminescence reaction product, coelenteramide anion, and a fluorescence anisotropy decay or rotational correlation time of 12 ns (20 °C), consistent with tight binding and rotation with the whole protein. A 34 ns correlation time combined with a translational diffusion constant and molecular brightness from fluorescence fluctuation spectroscopy all confirm that cgreGFP is an obligate dimer down to nanomolar concentrations. Within the dimer, the two chromophores have a coupled excited-state transition yielding fluorescence depolarization via FRET with a transfer correlation time of 0.5 ns. The 34 ns time of cgreGFP showed no change upon addition of a 1000-fold excess of Ca2+-discharged clytin, indicating no stable complexation below 0.2 mM. It is proposed that any bioluminescence FRET complex with micromolar affinity must be one formed transiently by the cgreGFP dimer with a short-lived (millisecond) intermediate in the clytin reaction pathway

Ligand binding and conformational states of photoprotein obelin

Many proteins require a non-covalently bound ligand to be functional. How ligand binding affects protein conformation is often unknown. Here we address thermal unfolding of the free and ligand-bound forms of photoprotein obelin. Fluorescence and far-UV circular dichroism (CD) data show that the various ligand-dependent conformational states of obelin differ significantly in stability against thermal unfolding. Binding of coelenterazine and calcium considerably stabilizes obelin. In solution, all obelin structures are similar, except for apo-obelin without calcium. This latter protein is an ensemble of conformational states, the populations of which alter upon increasing temperature

Oxygen activation of Apo-oberlin-Coelenterazine Complex

Ca2+-regulated photoproteins use a noncovalently bound 2-hydroperoxycoelenterazine ligand to emit light in response to Ca2+ binding. To better understand the mechanism of formation of active photoprotein from apoprotein, coelenterazine and molecular oxygen, we investigated the spectral properties of the anaerobic apo-obelin–coelenterazine complex and the kinetics of its conversion into active photoprotein after exposure to air. Our studies suggest that coelenterazine bound within the anaerobic complex might be a mixture of N7-protonated and C2(-) anionic forms, and that oxygen shifts the equilibrium in favor of the C2(-) anion as a result of peroxy anion formation. Proton removal from N7 and further protonation of peroxy anion and the resulting formation of 2-hydroperoxycoelenterazine in obelin might occur with the assistance of His175. It is proposed that this conserved His residue might play a key role both in formation of active photoprotein and in Ca2+-triggering of the bioluminescence reaction